Because the directors want to pander to the audience's desires and fetishes. Most movies now go through what seems to be some sort of checklist, and a love interest is usually at the top of the list. I personally like when movies don't include a love interest, makes it a bit more realistic and a lot less clichéd.
I hate when they add a love interest when the original story has none. They did that to the movie The Giver which is based on the book that had no love interest and they made the main character a teenager when he was only a child